The work presented in this paper extends a previous study of catalysts based on mixtures of MgO and Nd2O3 used in the oxidation coupling of methane (OCM) reaction. MgO-doped neodymia catalysts have been prepared by two different methods with various MgO loadings and their methane coupling performance compared with that of pure Nd2O3 Quite dramatic differences in catalytic performance have been observed between catalysts prepared by coprecipitation (Series A) and impregnation (Series B). Particularly striking is the highly beneficial effect on activity and Cz selectivity that the addition of 10 at.% MgO has on the Series B material. In both cases catalytic performance was related to the structural form of the material, Structural details for these catalysts have been obtained from high-resolution electron microscopy observations and powder X-ray diffraction. The results show that the microstructure of pure Nd2O3 is significantly different depending on the preparation route : Series A material contains Nd2O3 in various states ranging from areas having a high degree of crystallinity to extensive regions of semicrystalline/disordered material, whereas Series B Nd2O3 was almost exclusively in the hexagonal form, In general, MgO-doped neodymia catalyst materials have also been found to contain a complex mixture of Nd2O3 phases which for Series B materials became apparent after adding up to 3 at.% MgO, With a scanning transmission electron microscope, it has been determined that Mg2+ dissolution into the neodymia (up to 2.5 at.%) almost certainly takes place, This result in particular may explain the synergistic effect observed for OCM catalysts of this type,